This invention relates to a tappet for cam operation, for example, for a valve-activating arrangement in a reciprocating engine, and especially in an internal combustion engine, and a cam contact member therefor.
A tappet is disclosed in German Patent Specification No. 28 35 912. Its cam-contacting end consists of a circular plate or disc of highly elastic tool steel with both opposite sides of the plate being completely plane. If the actuating cam has a certain width or radius to the edge contacting the plate, the wear-reducing spring deflection of the cam-contacting plate, which is the object of the patented invention, is liable to be lost because the contact between the cam and tappet plate will be substantially linear and thus extend right across to the supporting or fixing edges of the tappet body for holding the plate. Consequently, a higher load cannot arise in the central, plate area of the cam-contacting face of the tappet than the plate-fixing edges of the tappet body can handle without excessive wear.
In addition, there is another drawback to the flat cam-contacting surface disclosed in the patent in that, when there is even a slight error in the alignment of the tappet, in other words, if the centerline of the tappet is not exactly perpendicular to the rotational axis of the camshaft--and this is a matter of a fraction of an angular minute--the plane-faced plate will bear unevenly on the cam. One plate-holding edge of the tappet body may then, again, bear on the cam to limit the wear reduction of the spring action of the plate.
A tappet having a spherical, cam-contacting surface is also known from, for example, German Patent Specification No. 12 09 802. The spherical, cam-contacting surface avoids the problem of uneven engagement with the cam because, if the tappet is misaligned relative to the rotational axis of the cam, the cam will merely engage a portion of the spherical surface adjacent that which it would have engaged with perfect alignment.
If a plane or spherical cam-contacting surface is made from hard, brittle sintered or ceramic-based materials, however, instead of the highly-elastic tool steel disclosed in the first-mentioned patent, for example, the high-frequency (Hertz) stresses that are liable to occur as the cam contacts the surface can dislodge or separate particles of the material. This produces excessive wear which was to be avoided with the use of the hard material. Such wear-producing, high-Hertz stresses are especially likely to occur if the cam-contacting surface is spherical due to the limited, point or line contact of the spherical surface with the cam. The successful use of such hard materials in a spherical cam-contacting surface is, therefore, additionally unexpected as is, still further, any wear-reducing spring-like deflection action from such hard materials.